The use of mixed bed ion exchangers is prior art in the desalting of aqueous solutions. Surface interactions between the cationic and anionic components of the mixture lead to the formation of agglomerates (“clumping”) and thereby to poor flow behaviour. Furthermore, the performance of such mixed beds is unsatisfactory. In addition the agglomeration prevents effective regeneration of the ion exchange resins for which the mixture must be fractionated as completely as possible into its components.
To prevent the unwanted clumping in mixed beds, diverse methods have been described such as, for example a) the treatment with water-insoluble ion exchange particles in U.S. Pat. No. 4,347,328 where high amounts used and high washwater volumes are required in order to remove excess ion exchange particles again or else b) the treatment with water-soluble resinous polyelectrolytes according to U.S. Pat. Nos. 2,961,417, 3,168,486, 5,902,833, 6,060,526 or EP-A 1 291 083. The water-soluble polyelectrolytes for treating the anion exchange component are water-soluble resinous polyelectrolytes such as polymeric acrylic or methacrylic acid (derivatives), but preferably sulphonated polyvinylaromatics, and very particularly preferably polystyrenesulphonic acid (PSS). Depending on the mean molar weight of the PSS, optimum amounts used can be determined according to diverse criteria. For mean molar weights of 5000 to 1 000 000 g/mol, the preferred amounts used are in the range from 10 to 800 mg/liter of anion exchanger.
Separability of the mixed beds, however, is only achieved on simultaneous treatment of the cation exchange component as described, for example, in EP-A 1 291 083. In this case also, use is made of water-soluble polyelectrolytes, for example polyvinylaromatic quaternary ammonium salts, polyvinylaromatic amino acid salts and polyvinylpyridinium salts having mean molar weights of 5000 to 1 000 000 g/mol.
The above processes all have the disadvantage that the polyvinylaromatic acids or ammonium compounds are only accessible in the suitable molar mass range with relative difficulty and that to achieve good separability in the mixed bed both charge components must be treated.
An object of the present invention was to overcome the described disadvantages.